/Engine/Source/Runtime/Engine/Classes/Engine/AssetManager.h
Unloads a list of Primary Assets that were previously Loaded. If the only thing keeping these assets in memory was a prior Load call, they will be freed.
Number of assets unloaded
virtual int32 UnloadPrimaryAssets ( const [TArray](API\Runtime\Core\Containers\TArray)< [FPrimaryAssetId](API\Runtime\CoreUObject\UObject\FPrimaryAssetId) > & AssetsToUnload )